1. Field of the Invention
The invention relates to an integrated amplifier comprising an auxiliary amplifier which includes a first long-tailed pair circuit having an input for receiving an auxiliary voltage, a signal amplifier which includes at least a second long-tailed pair circuit having an input for receiving a signal voltage to be amplified, and adjusting means for adjusting, via a tail current from at least the second long-tailed pair circuit, a gain factor of the signal amplifier in response to the value of an output current of the first long-tailed pair circuit.
An integrated amplifier of this type may be used, for example, as a microphone preamplifier in a transmission IC for wire telephony. In that field there is an important requirement that the gain factor be accurately defined.
2. Description of the Prior Art
An integrated amplifier as described in the opening paragraph is known from U.S. Pat. No. 4,489,282. In the amplifier described in that document the output current of the first long-tailed pair circuit is compared with an external reference current. The tail current of one or more long-tailed pair circuits in the signal amplifier is adjusted on the basis of the difference between these currents. The external reference current is dependent on an off-chip resistor. External resistors can be obtained with any desired accuracy. This provides great accuracy of the gain factor to be adjusted. A drawback of the amplifier described in that document is that it is not fully integrable and, therefore, relatively expensive due to the presence of the off-chip resistor.